Process for the continuous polymerization of lactams with static mixers

ABSTRACT

The invention relates to a process for the continuous polymerization of caprolactam in a reactor, wherein a reactor in which the first 60% of the reactor volume is equipped if possible completely but at least to one-third with static mixing installations is used.

This is a continuation of application Ser. No. 620,312, filed Oct. 3,1975, now abandoned.

This invention relates to a process for the continuous preparation ofpolyamides, preferably from ε-caprolactam as the main or only component,to which, for example, matting agents, colouring agents or light andheat stabilizing substances may be added.

It is known to carry out the continuous polymerisation of polyamides inapparatus of various constructions. In the simplest case, the reactorconsists of a simple, vertical tube into which caprolactam, for example,is introduced at the top and from which polymer is removed at the bottomwhile water is driven off as steam at the top. The desire to control therate of removal of water in accordance with reaction kineticconsiderations has led to the development of multistage tubes, in whichthe various stages may be telescoped into each other. In the lattercase, ascending and descending streams of melt are obtained.

Other devices provided on and in the reactor have the object ofimparting a desired temperature profile to the caprolactam which isundergoing polymerisation. This is achieved by simply heating the wallof the reactor or by installing heat exchangers, for example in the formof plates or nests of tubes arranged parallel to the direction of flow.It has been found suitable to carry out the initial phase of thereaction at temperatures above 205° C. and the final phase below 250°C., that is to say heat is applied to the top of a VK tube (e.g. theVK-process is described in "Kunststoffhandbuch" Vol. VI, Carl HauserVerlag Munich (1966) page 191) and removed in the lower part by asuitable heat exchanger. It is important to ensure that cooling sets inonly when the conversion of caprolactam, which, as is well known, is apolyaddition reaction, has been substantially completed.

Other developments of the process have the subject of ensuring more orless thorough mixing of the caprolactam which is to be polymerised,either in order to stir additives into the caprolactam in the initialphase or in order to obtain a homogeneous melt. The mechanical stirrersof various construction frequently used for this purpose areunsatisfactory, firstly because they are liable to give trouble due todriving and sealing problems and secondly because they do not help toproduce the required plug flow. Among the various solutions to overcomethis difficulty, it has been proposed to use cascades of stirrer vesselswhich contribute to the formation of plug flow and the more so thelarger the number of vessels arranged in series. This solution, however,resulted in systems which were very complicated and therefore liable togive trouble so that it was not possible to ensure an absolutelyconstant reaction process which is necessary in order to obtain highquality products.

It is also known to influence the flow in a VK tube by installingvarious devices in the interior of the tube, for example V-shaped orperforated baffle plates, conical hollow bodies or concentricallyarranged annular surfaces extending parallel to the direction of flow toact as braking surfaces with a view to improve the flow of the melt, butnone of these devices has been found to be completely satisfactory.

The so-called static mixers which have recently become known provide acombination of a mixing action and an action which influences the flow.

It is an object of the present invention to improve the process for thecontinuous polymerisation of ε-caprolactam so that not only a thoroughlymixed melt flowing as a plug flow is obtained but also the output is atleast 20% higher than can be achieved in prior art processes.

This problem is solved by carrying out the continuous polymerisation ofcaprolactam in a reactor in which the first 60% of the reactor volume isequipped if possible completely but at least to one-third with suitablestatic installations.

According to the invention there is provided a process for thecontinuous polymerisation of caprolactam in a reactor, wherein at leastone-third of the first 60% of the reactor volume is equipped with staticmixing installations.

The reactor tubes of this invention contain at least one mixing insertconsisting of webs inclined at an angle to each other. In oneembodiment, these webs intersect, the inserts consisting of at least twoslotted plates set at an angle to each other inside the housing andintermeshing through the slots. In another embodiment, the webs set atan angle to each other do not intersect but contact each other along anedge to form a gable with its axis perpendicular to the axis of thetube.

As the stream of material encounters the webs, it is split up into anumber of individual streams separated from one another in time andspace due to the oblique positioning of the webs. On the downstream sideof the webs there is a flow gradient in the transverse direction whichensures efficient exchange between the individual streams. Sincedivision of the stream of material into separate streams is staggered intime and space, mixing also takes place in the direction of flow andthis is superimposed by radial flow components. With the transversemixing which can be achieved it is possible to obtain a goodapproximation to a plug flow profile so that a narrow spectrum of dwelltimes within the reactor can be obtained, which is advantageous forcaprolactam polymerisation. The inclination of the webs to the directionof the oncoming flow and to the wall of the tube may also be in morethan one dimension. Additional mixing effects are thereby obtained.

According to one particular embodiment, the mixing insert comprises aplurality of pairs of plates which have a comb-like structure, and theplates are arranged in two rows so that the webs of at least two layersof one row intersect the webs of at least one layer of the other row.

Several lines of intersection are thereby produced, along which thepartial streams are further subdivided.

The plates of each row are preferably arranged parallel to each other.This arrangement provides the possibility of more rational manufactureof the plates.

According to one particularly advantageous embodiment, several mixinginserts are provided and the webs of the pairs of plates of the mixinginserts are set at an angle to each other. This angle may be, forexample, 90°. With an angle of 90°, the required length of the apparatusis particularly short because the plates of the pairs of plates or theplates of the outer pairs of plates of adjacent mixing inserts can thenbe pushed far into the gaps between the pairs of plates of the adjacentinsert. The angular displacement between the mixing inserts in a rowresults in a subdivision in space of the individual streams which areproduced by the webs. With this embodiment, very powerful mixing effectscan be obtained with only a few mixing inserts arranged in a row.

The plates are preferably in the form of combs which have a generallyelliptical outline, and in order to improve the flow in the marginalzones of the tube, the web connecting part is shifted from the wall ofthe tube towards the centre of the tube. In a rational manufacturingprocess, this can be achieved by punching out. It goes without sayingthat other manufactured processes are also suitable for manufacturingthe mixing inserts, for example large mixing inserts can be produced bywelding. The plates may be constructed, for example, by fixing the websto a closed ring. The webs and slots are preferably arranged parallel tothe main axis of the plates. This constitutes another advantage withregard to manufacture of the plates.

According to a particular embodiment, the webs have a specialcross-sectional profile, for example they may be triangular, tear-shapedor elliptical. Special flow effects which ensure vigorous mixing in thetransverse direction, for example, are obtained in the case of atriangular section if one edge is at an angle against or in thedirection of flow. The webs may also have a hollow profile. In thatcase, they may be traversed by a heating fluid, for example, so thatthey also perform a heat transfer function.

If it is desired to vary the velocity of flow of the medium at thecentre of the tube as compared with the wall, it is advantageous to varythe width of the webs. The flow is then displaced further towards thecentre or towards the periphery according to whether the webs are wideror narrower towards the centre or the periphery.

The various possible variations of the apparatus for carrying outcontinuous caprolactam polymerisation afford the constructional engineerwide scope for optimising the apparatus. In particular, the mixinginserts can be constructed from variously formed webs, or variouslyformed mixing inserts with similar webs can be arranged behind oneanother in a suitable sequence. In this way it is possible to adapt themixing inserts to special operating factors such as the flow velocity,the viscosity and the residence time of the reactants in the varioussections.

Various embodiments of the apparatus according to the invention areshown purely diagrammatically in the accompanying drawings and describedbelow.

FIGS. 1 to 5 represent various constructional examples of the apparatuswith different arrangements of the mixing inserts or plates and

FIG. 6 shows an example in which the webs do not intermesh.

In FIGS. 1 to 6, similar parts have been indicated by reference numeralshaving the same digits in the unit place preceded by a digitrepresenting the number of the figure.

FIG. 1 shows several mixing inserts 12 arranged behind one another atangles of 90° to each other inside a tube 11. The mixing inserts areformed by intermeshing comb-like plates.

FIG. 2 shows mixing inserts 22 arranged in a tube 21. Each insertconsists of five pairs of plates 23, 23'. The mixing inserts 22 aredisplaced by an angle of 90° relative to each other. Plates 23, 23' havethe same configuration as in FIG. 1.

The mixing inserts 32 arranged in tube 31 in FIG. 3 are similar in formto those shown in FIG. 2 but in this case the oblique webs are inaddition inclined to their longitudinal axis.

FIG. 4 shows a tube 41 containing mixing inserts 42 which merge intoeach other in that the plates 43, 43' in each case extend throughseveral intersecting plates. The plates are arranged that there arealways two parallel plates 43, 43' set close to each other while thedistance between these two plates and the next following two plates isabout twice as great as the distance of the first two plates to eachother.

In the embodiment shown in FIG. 5, a tube 51 contains mixing inserts 52in which plates 53, 53' intersect not only each other but also theplates of adjacent mixing inserts 52, as in the example shown in FIG. 4,so that individual mixing inserts cannot be exactly defined. In thisembodiment, importance is attached to placing the lines of intersectionoutside the central axis of the tube 51.

Alternatively, the lines of intersection may be dispensed with, as inthe embodiment shown in FIG. 6, and the individual webs may be connectedalong their edges to form V-shaped inserts. In this embodiment, all theadjacent layers of webs may be displaced from each other in any waydesired.

FIGS. 7 to 10 show diagrammatically reaction tubes for carrying out theprocess according to the invention. Similar parts have again beenindicated by the same unit digits preceded by the number of the figure.

71, 81 91 and 101 indicate the lactam supply. 72, 82, 92, and 102indicate a condenser. The reaction tubes 73, 83, 93 and 103 are equippedwith static installations 74, 84, 94, and 104. In FIG. 9, a hydrolysisapparatus 97 equipped with a stirrer 95 is attached to the upstream endof the reaction tube. In FIG. 8, a stirrer 85 is provided inside thereaction tube. In this case, the lactam is preheated at 86.

It will, of course, only be possible to apply the previously describedapparatus in accordance with the invention in cases where the stirrer orheat exchanger does not take up the entire volume of the reactor. It is,of course, possible and in many cases advisable, in the interests ofobtaining a homogeneous melt with a plug flow to instal static unitsalso in the last 40% of the reactor volume, but this is not necessaryfor the purpose of the invention. On the other hand, it would be quiteinsufficient for the purpose of the invention to equip only the last 40%of the reactor volume with these installations.

The known packing mixers which consists of a plurality of parallel,intersecting channels open to each other on one side are also suitablefor carrying out caprolactam polymerisation in accordance with theinvention, although for tubes of large diameter, in the case describedhere of the order of up to 1500 mm, these mixers are not sufficientlyeconomical owing to the manufacturing costs and the high cost of thinsheet metal. Moreover, the pressure drop depends to a very great extenton the selected geometrical configuration of the channel, which is insome cases disadvantageous.

Static mixers which have to be very long in order to achieve a goodmixing effect, for example one known embodiment consisting of acombination of alternately left-handed and right-handed helicalelements, cannot be used in the process of the invention, with respectto the big diameters of VK-tubes. For special construction, e.g. bydividing the cross section of big VK-tubes into many parallel tubes ofsmaller diameters, this mixing unit could be suitable for carrying outcaprolactam polymerisation, but with high costs of construction.

The effect of the present invention is illustrated by a comparison ofthe extract values of a polycaprolactam which has been obtained in aconventional polymerisation tube in Example A with those of apolycaprolactam which has been obtained according to Example B in a tubewhich is completely identical with the tube used in Example A but inaddition is equipped with the installations according to the invention.The quantities of chain regulator and water and the reactiontemperatures employed are also identical in the two cases. The lactamoutput from reactor A is 11.0 tons per day and from reactor B 12.1 tonsper day. The extract values were obtained by the methanol method.

    ______________________________________                                        Polymerisation tube A                                                                           Polymerisation tube B                                       11.0 tons per day output                                                                        12.1 tons per day output                                    ______________________________________                                        11.95 % extract   9.54 % extract                                              12.01 % extract   9.84 % extract                                              12.28 % extract   9.11 % extract                                              12.00 % extract   9.85 % extract                                              11.90 % extract   9.78 % extract                                              11.92 % extract   9.99 % extract                                              12.21 % extract   9.23 % extract                                              ______________________________________                                    

For kinetic reasons, caprolactam conversions of 90% and more can beachieved in hydrolytic caprolactam polymerisation. It is clear that thisconversion can easily be achieved in a reaction tube B operated inaccordance with the invention whereas the conversion rate in the normalreactor is in the region of 88% although the caprolactam output is about10% lower.

We claim:
 1. A process for continuous polymerization of caprolactamcomprising heating caprolactam in a reactor under conditions sufficientto polymerize it to resinous polycaprolactam wherein said reactorcomprises a reactor tube having a first end where caprolactam is fed anda second end where resinous polycaprolactam is removed, said reactorcontains static mixing means in at least 30% of the first 60% of thereactor volume and said first 60% of reactor volume is measured fromsaid first end.
 2. Process according to claim 1, wherein the reactortube used is one in which the first 30% of the reactor volume contains,as static mixing means, a mixing insert in the form of a pair of plateswhich have webs inclined at an angle to each other and to the axis ofthe reactor tube, the webs of the two layers either intermeshing likeforks or abutting against each other along their edges in the centre ofthe tube to form a roof gable with the gable edge situated perpendicularto the axis of the tubes.
 3. Process according to claim 1, wherein thereactor tube used is one in which the first 60% of the reactor volumecontains, as static mixing means, a mixing insert comprising severalcomb-like pairs of plates, the plates being arranged in two rows so thatthe webs of two layers of one row intersect the webs of at least onelayer of the other row.
 4. Process according to claim 3, wherein thereactor tube used is one in which the first 60% of the reactor volumecontains, as static mixing means, mixing insert in which the plates ofeach row are arranged parallel to each other.
 5. Process according toclaim 2, wherein the reactor tube used is one in which the first 60% ofthe reactor volume contains, as static mixing means, a mixing insertconsisting of hollow section webs traversed by a heating medium, thewebs being arranged at an angle to each other and to the axis of thereactor tube.
 6. Process according to claim 3, wherein the reactor tubeused is one in which the first 60% of the reactor volume contains, asstatic mixing means, a mixing insert consisting of hollow section websthrough which a heating medium flows, the webs of two layers of one rowintersecting the webs of at least one layer of the other row.
 7. Processaccording to claim 2, wherein the reactor tube used is one in which thefirst 60% of the reactor volume contains, as static mixing means, amixing insert in which the individual webs are welded and adjacent websare arranged in any desired position in relation to each other.
 8. Theprocess of claim 1 in which the caprolactam conversion andpolycaprolactam output obtained are higher than obtained with the samereaction conditions in a tube reactor without said static mixing means.